1. Technical Field
This invention relates to a servo-control apparatus and, more particularly, to a servo-control apparatus capable of performing a backlash correction rapidly and of reducing quadrant projections produced as a result of friction and backlash.
2. Background Art
In order to correct for backlash of gears, or the like, when the direction of rotation of a servomotor controlling the feed shaft of a machine tool is reversed, or in other words, when the direction of travel of the movable element of the machine is reversed, preset backlash correction data is applied to position error data to generate a velocity command, and the rotation of the servomotor is controlled based on the velocity command. However, since the backlash correction data acts upon a velocity loop through a predetermined position gain K.sub.P, the machine does not move immediately because of a delay in the servo-system. As a consequence, a quadrant projection is produced at the location where the direction of machining is reversed.
Accordingly, in order to diminish the effects of backlash and friction and reduce projections at quadrant changeover points at the time of cutting, it has been proposed to apply a suitable amount of acceleration (referred to as "backlash acceleration") to the velocity command for a prescribed period of time. However, when an NC unit effects backlash correction at the same time that the backlash correction data is delivered to the servo side, the motor is not completely reversed in direction owing to the delay between the command and the servo-system. The unfortunate result is that bite-in is caused on the inner side. In actuality, therefore, the direction in which the motor rotates is monitored and backlash acceleration is applied at the timing of the reversal in direction of rotation. When control of backlash acceleration is carried out at the time of the reversal in motor direction, the occurrence of quadrant projections caused by backlash and friction in the machine system can be reduced considerably.
There are cases where the effect of a sudden change in velocity at the end of acceleration shows up at the cut surface in the form of inadequate smoothness if the amount of backlash acceleration is made constant over the entire period of application. Accordingly, it has been proposed to enlarge the amount of backlash acceleration at the start of acceleration in order to prevent inadequate acceleration, and thereafter reduce the amount of acceleration gradually with the passage of time to prevent a sudden change in velocity when backlash acceleration ends.
However, the backlash acceleration algorithm itself is open-loop control, and it is required that parameters (the amount of backlash acceleration, backlash acceleration time, etc.) be set accurately for each and every machine.
In addition, optimum parameters change depending upon cutting velocity, workpiece weight, temperature, the condition of the cut surface, etc. Consequently, even if quadrant projections can be made very small under certain conditions, these projections can enlarge or cut-in can result on the inner side when cutting is performed under other conditions.